1. Technical Field
The invention relates to an objective lens, an optical pickup device, and an optical recording/reproducing apparatus. Specifically, the invention relates to an objective lens for use in an optical pickup apparatus for converging light emitted from a light source onto an optical recording medium to perform at least one of operations of recording and reproducing information, an optical pickup device having the objective lens, and an optical recording/reproducing apparatus equipped with the optical pickup device.
2. Description of the Related Art
Generally, various optical recording media such as DVD (digital versatile disk) and CD (compact disk) have been used in order to record audio information, video information, or data information for computer. As an amount of information to be handled rapidly increases, an increase in storage capacity density of an optical recording medium has been strongly demanded. It has been known that a decrease in wavelength of used light and an increase in numerical aperture (hereinafter, it is referred to as an NA) of an objective lens for an optical pickup device are effective to increase the storage capacity density of an optical recording medium. Recently, for a BD (blu-ray disk), a semiconductor laser having an output wavelength of about 405 nm is used as a light source and an objective lens having an NA of 0.7 or more is used. Such a blu-ray disk (hereinafter, it is referred to as BD) having a capacity of 25 GB per one layer of one side has been widely used in practice. In the specifications of the BD, an NA and a thickness of a protection layer of an optical recording medium are set totally different from values of DVD and CD. In the current specification, an NA is 0.85, and a thickness of a protection layer is 0.1 mm.
However, in future, as might be expected, an increase in density will be demanded more and more, but it may be hard to satisfy this demand by promoting a decrease in wavelength. This is because optical transmittance of lens materials is rapidly reduced in the range of a wavelength less than 350 nm and thus, it is hard to obtain sufficient optical efficiency in practice. For this reason, the other way for achieving high storage capacity density is to further increase an NA of the objective lens.
When an objective lens with a high numerical aperture (hereinafter, it is referred to as a high NA) for an optical pickup device is designed, a single lens structure is effective to solve problems such as an increase in process number at the time of assembly, deterioration in production efficiency, and an increase in cost. For example, known objective lenses for an optical pickup device having a relatively high NA single lens are described in JP 2001-324673 A (corresponding to U.S. Pat. Nos. 6,411,442 and 6,512,640) and Japanese patent No. 3712628 (corresponding to U.S. Pat. No. 6,744,568).
JP 2001-324673 A describes an objective lens that can achieve excellent image height characteristics by setting a ratio of a thickness d of the lens on the optical axis to a focal length f thereof in a predetermined range. Japanese patent No. 3712628 describes an aspheric objective lens having a light source side surface that is formed as a convex surface and an optical recording medium side surface that is formed in a shape substantially flatter than the light source side surface.
Meanwhile, in the high-NA objective lens for an optical pickup device, it is difficult to secure a working distance (WD) that can prevent collision between the lens and an optical recording medium while maintaining compactness of the lens system. In this point of view, the objective lens described in JP 2001-324673 A has a room for improvement. Also, in focusing and tracking controls, it is preferable to drive a lens at higher speed. For this reason, it is also required to reduce a weight of a lens.
However, in the objective lens described in Japanese patent No. 3712628, a refractive index of the lens component material is set high, and thus a weight of lens becomes large. In lens material, particularly in glass material, when a refractive index thereof is high, a specific gravity generally tends to increase. For example, a specific gravity of a material having a refractive index of about 1.60 is about 2.8, but a specific gravity of a material having a refractive index of about 1.85 is about 4.3. Accordingly, assuming that volumes of lenses are the same, a weight of a lens formed of a material having a refractive index of about 1.85 is 1.5 times as large as a weight of a lens formed of a material having a refractive index of about 1.60. This causes a problem in that it is difficult to drive a lens at a high speed in focusing and tracking controls since the weight of the lens described in Japanese patent No. 3712628 becomes large in any cases.